Missing Solar Neutrinos Revealed
Posted: Monday, June 18, 2001
First Results from the (SNO) Sudbury Neutrino Observatory Explain the Missing Solar Neutrinos and Reveal New Neutrino Properties
Physicists from Canada, the UK and the US are today announcing that their first results provide a solution to a 30-year old mystery- the puzzle of the missing solar neutrinos. The Sudbury Neutrino Observatory (SNO) finds that the solution lies not with the Sun, but with the neutrinos, which change as they travel from the core of the Sun to the Earth.
Neutrinos are elementary particles of matter with no electric charge and very little mass. There are three types: the electron-neutrino, the muon-neutrino and the tau-neutrino. Electron-neutrinos, which are associated with the familiar electron, are emitted in vast numbers by the nuclear reactions that fuel the Sun. Since the early 1970s, several experiments have detected neutrinos arriving on Earth, but they have found only a fraction of the number expected from detailed theories of energy production in the Sun. This meant there was something wrong with either the theories of the Sun, or the understanding of neutrinos.
"We now have high confidence that the discrepancy is not caused by problems with the models of the Sun but by changes in the neutrinos themselves as they travel from the core of the Sun to the earth," says Dr. Art McDonald, SNO Project Director and Professor of Physics at Queen's University in Kingston, Ontario. "Earlier measurements had been unable to provide definitive results showing that this transformation from solar electron neutrinos to other types occurs. The new results from SNO, combined with previous work, now reveal this transformation clearly, and show that the total number of electron neutrinos produced in the Sun are just as predicted by detailed solar models."
The SNO scientists present their first results today in a paper submitted to Physical Review Letters and in presentations at the Canadian Association of Physicists Annual Conference at Victoria, B.C. and at SNO Institutions in the U.S. and the U.K. "It is incredibly exciting, after all the years spent by so many people building SNO, to see such intriguing results coming out of our first data analysis - with so much more to come." says UK Co-spokesman Prof. David Wark of the Rutherford/Appleton Laboratory and the University of Sussex.
The determination that the electron neutrinos from the Sun transform into neutrinos of another type is very important for a full understanding of the Universe at the most microscopic level. This transformation of neutrino types is not allowed in the Standard Model of elementary particles. Theoreticians will be seeking the best way to incorporate this new information about neutrinos into more comprehensive theories.
The direct evidence for solar neutrino transformation also indicates that neutrinos have mass. By combining this with information from previous measurements, it is possible to set an upper limit on the sum of the known neutrino masses. "Even though there is an enormous number of neutrinos in the Universe, the mass limits show that neutrinos make up only a small fraction of the total mass and energy content of the Universe." says Dr. Hamish Robertson, U.S. Co-Spokesman and Professor of Physics at the University of Washington in Seattle.
The SNO detector, which is located 2000 meters below ground in INCO's Creighton nickel mine near Sudbury, Ontario, uses 1000 tonnes of heavy water to intercept about 10 neutrinos per day. The results being reported today are the first in a series of sensitive measurements that SNO is performing. From this initial phase, the SNO scientists report on an accurate and specific measurement of the number of solar electron neutrinos reaching their detector, by studying a reaction unique to heavy water where a neutron is changed into a proton. They combined these first SNO results with measurements by the SuperKamiokande detector in Japan of the scattering of solar neutrinos from electrons in ordinary water (offering a small sensitivity to other neutrino types), to provide the direct evidence that neutrinos oscillate.
At the beginning of June the SNO scientists began the next phase of their measurements, by adding salt to the heavy water, to study another neutrino reaction with deuterium that provides a large sensitivity to all neutrino types. Their further measurements can address the transformation of neutrino type with even greater sensitivity, as well as studying other properties of neutrinos, of the Sun and supernovae.
Background Information on the Sudbury Neutrino Observatory
The Sudbury Neutrino Observatory is a unique neutrino telescope, the size of a ten-storey building, 2 kilometers underground in INCO's Creighton Mine near Sudbury Ontario planned, constructed and operated by a 100-member team of scientists from Canada, the United States and the United Kingdom. Through its use of heavy water, the SNO detector provides new ways to detect neutrinos from the sun and other astrophysical objects and measure their properties. For many years, the number of solar neutrinos measured by other underground detectors has been found to be smaller than expected from theories of energy generation in the sun. This has led scientists to infer that either the understanding of the Sun is incomplete, or that the neutrinos are changing from one type to another in transit from the core of the Sun.
The SNO detector has the capability to determine whether solar neutrinos are changing their type en-route to Earth, thus providing answers to questions about neutrino properties and solar energy generation.
The SNO detector consists of 1000 tonnes of ultra-pure heavy water enclosed in a 12-meter diameter acrylic plastic vessel, which in turn is surrounded by ultra-pure ordinary water in a giant 22-meter diameter by 34-meter high cavity. Outside the acrylic vessel is a 17-meter diameter geodesic sphere containing 9456 light sensors or photomultiplier tubes, which detect tiny flashes of light emitted as neutrinos are stopped or scattered in the heavy water. The flashes are recorded and analyzed to extract information about the neutrinos causing them. At a detection rate on the order of 10 per day, many days of operation are required to provide sufficient data for a complete analysis. The laboratory includes electronics and computer facilities, a control room, and water purification systems for both heavy and regular water.
The construction of the SNO Laboratory began in 1990 and was completed in 1998 at a cost of $73M CDN with support from the Natural Sciences and Engineering Research Council of Canada, the National Research Council of Canada, the Northern Ontario Heritage Foundation, Industry, Science and Technology Canada, INCO Limited, the United States Department of Energy, and the Particle Physics and Astronomy Research Council of the UK. The heavy water is on loan from Canada's federal agency AECL with the cooperation of Ontario Power Generation, and the unique underground location is provided through the cooperation and support of INCO Limited.
Measurements at the SNO Laboratory began in 1999, and the detector has been in almost continuous operation since November 1999 when, after a period of calibration and testing, its operating parameters were set in their final configuration.
Further information about the SNO detector can be found on the SNO Detector page.
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